Inverter circuit

ABSTRACT

The invention relates to an inverter circuit which includes at least one pair of series connected transistors. The transistors of each pair are switched alternately on and off with a short dead period therebetween to prevent short circuiting. Rapid switching is accompanied by power being converted in each transistor on being switched off which is harmful to the transistor. Known auxiliary circuits disposed respectively parallel with the transistors include capacitors to slow the rise of the collector voltage relative to the drop in emitter current and this provides some protection against damage. Additional protection is attained by providing a choke coil in series with each pair of transistors which retards the charging of the capacitors and the transistor charging current is thereby kept low to prevent or minimize transistor damage.

This is a continuation patent application of Ser. No. 816,810, filedJuly 18, 1977, now abandoned.

The invention relates to an inverter circuit comprising at least onepair of transistors which are in series between two D.C. voltage supplyleads and between which an output branches off to the consumer, thetransistors being alternately brought to the conductive state and eachlying in parallel with an auxiliary circuit comprising a condenser inseries with a parallel circuit consisting of a resistor and a diodepoled in the same sense as the pass direction of the transistors.

In a known inverter circuit, such an auxiliary circuit, which is alsotermed a R-C-D protective circuit, is provided for the following reason.If in a transistor without the auxiliary circuit there is no controlvoltage at the base, the emitter current drops back to zero suddenly butwithin a finite time, whilst the collector-emitter voltage increasesfrom a low value to the full size, this again being sudden but within afinite time. Consequently, power is converted in the transistor that canlead to destruction of the transistor in the course of time. By usingthe auxiliary circuit, the rise in the collector-emitter voltage isretarded in relation to the drop in the emitter current. Consequentlythe power converted in the transistor on `switching off` can be kept solow that it is no longer harmful.

Surprisingly, it has been found that despite careful dimensioning of theauxiliary circuit damage occurs to the transistors if these areconnected in series in pairs in an inverter. The invention is thereforebased on the problem of providing an inverter circuit of theaforementioned kind in which the transistors that are used are moresafely protected against damage.

This problem is solved according to the invention in that asupplementary circuit containing a choke is provided in series with thepair of transistors and associated auxiliary circuits.

If in an inverter circuit the transistors are alternately brought to theconductive state, a certain dead period must be provided between`switching off` the first transistor and `switching on` the secondtransistor so that a short circuit path can on no account occur throughboth transistors between the two supply leads. For safety reasons, thisdead period should be chosen to be too long rather than too short.Within the dead period, the D.C. voltage is distributed so that aboutU/2 is applied to each condenser of the auxiliary circuit. If the onetransistor is then brought to the conductive state, the full voltage Uwill suddenly be applied to the condenser associated with the othertransistor. This leads to a correspondingly high charging current whichis supplied through the conductive transistor and leads to its rapidbreak-down. If the path of this charging current is provided with asupplementary circuit containing a choke in accordance with theinvention, charging of the condenser will take much longer and thetransistor will not be damaged.

Another advantage of the supplementary circuit resides in the fact thata single supplementary circuit will in any case suffice for twotransistors with associated auxiliary circuits but possibly for far moretransistors with associated auxiliary circuits. The choke can also haveadditional functions.

Normal operation of the inverter circuit is in no way affected by thechoke because its D.C. resistance is very low. It is, however, desirableto have a branch which bridges the choke and contains a diode poledopposite to the pass direction of the transistors. In this way oneobtains a circuit through which the choke current can fade out duringthe dead period.

A resistor may be in series with the diode in the bridging branch. Thispermits the time constant of the fading-out process to be set. If theresistor is not too large, it may also be in series with the choke andthis series circuit bridged by the diode.

Further, it is not necessary for the supplementary circuit to besymmetrical. It is even an advantage if it is provided between one ofthe D.C. supply leads and one end of the series circuit of a pair oftransistors. In this way there will be no influence on the regionbetween the two transistors on the output side and it can be ofconventional form.

If two or more pairs of transistors are provided, it is advisable forthe supplementary circuit to be in a lead common to the pairs. Onesupplementary circuit will then suffice for all transistors andassociated auxiliary circuits.

The invention will now be described in more detail with reference to theexample shown in the drawing, wherein:

FIG. 1 is a circuit diagram for a two-phase inverter with earthed centrepoint and,

FIG. 2 is a circuit diagram for a three-phase inverter.

In FIG. 1 a regulatable voltage supply 1 is provided from which thereextend a supply lead 2 for the positive D.C. voltage, a supply lead 3for the negative D.C. voltage and an earthed central connection 4.

A first transistor Tr1 and a second transistor Tr2 are in series withtheir collector-emitter path. The base-emitter path is bridged by aresistor R1 or R2. If a control voltage is applied to two control signalterminals K1 and K2, one of the transistors Tr1 will become conductive.If the control signal is absent, it returns to the blocked condition.The same applies to the transistor Tr2, except that the control signalsare applied alternately and there is a dead period between them.

An output 5 branches off between the transistors and between the outputand earth an A.C. consumer 6 can be connected.

Parallel to the transistor Tr1 there is an auxiliary circuit 7 andparallel to the transistor Tr2 an auxiliary circuit 8. The auxiliarycircuit 7 consists of a series circuit of a condenser C1 and theparallel circuit of a resistor R3 and a diode D1 which has the same passdirection as the transistor Tr1. The auxiliary circuit 8 has the sameconstruction and comprises a condenser C2, a resistor R4 and a diode D2.

In series with the circuit as described comprising the pair oftransistors Tr1 and Tr2, there is a supplementary circuit 9 having achoke L and a bridging branch with a resistor R5 and a diode D3 of whichthe pass direction is opposite to that of the transistors.

Assuming that in operation the transistor Tr1 is conductive and thetransistor Tr2 is blocked, then in this condition the condenser C1 ischarged to a very low voltage and the condenser C2 to almost the fullD.C. voltage U. If, now, the transistor Tr1 is blocked, the emittercurrent will fade out comparatively rapidly. On the other hand, thecollector-emitter voltage will rise more slowly because this takes placein response to the charge on the condenser C1. Accordingly, the powerconverted in the transistor Tr1 on switching off is so low that thetransistor cannot be damaged.

Within the dead period there is a distribution of the voltage such thatapproximately U/2 is applied to both condensers C1 and C2. If, now, thetransistor Tr2 becomes conductive then, in the absence of thesupplementary circuit 9, substantially the full D.C. voltage U would beapplied to the condenser C1. A strong charging current would flowthrough the transistor Tr2 and damage same. By reason of the choke L inthe supplementary circuit 9, the charging of the condenser C1 isretarded. The corresponding charging current can be kept so low that thetransistor Tr2 does not become damaged.

On blocking the transistor Tr2, the conditions are repeated as alreadydescribed in conjunction with blocking of the transistor Tr1. Thecurrent then still flowing in the choke L is short circuited through theresistor R5 and the diode D3 so that the choke current drops rapidlyand, on switching the transistor Tr1 on, the choke is again able to keepthe charging current sufficiently low, this time for the condenser C2.

In the FIG. 2 embodiment, the same reference numerals as in FIG. 1 areused for the same components. Since this is a three-phase inverter,three pairs of transistors Tr1, Tr2 and Tr3, Tr4 and Tr5, Tr6 areprovided, in each case with the associated auxiliary circuits such as 7and 8. All three pairs of transistors have a common protective circuit9.

In this protective circuit, a measuring resistor R6 is in series withthe choke L. A measuring signal i corresponding to the direct currentflowing through the inverter can be tapped from this resistor. Theseries circuit L and R6 is bridged by the diode D3.

Three outputs 5U, 5V and 5W are provided to which a three-phase consumersuch as an asynchronous motor can be connected. Further, there arefree-running diodes D4-D9 which operate in the usual manner. Thefree-running diodes D4, D6 and D8 are provided directly between arespective output and the positive supply lead 2, the free-running diodeD5, D7 and D9 are provided between the outputs and a point 10 of thenegative supply lead 3 located between the voltage supply 1 and thesupplementary circuit 9.

I claim:
 1. An inverter circuit comprising a voltage supply and a pairof supply leads, at least one pair of series connected transistors and asupplementary circuit in series with said transistors extending betweensaid supply leads, a consumer load connected to the junction of eachpair of said transistors, an auxiliary circuit in parallel with each ofsaid transistors including a capacitor in series with a parallel circuitof a resistor and a diode, said diode being poled to pass current in thesame direction as the associated one of said transistors, meansoperating the two transistors of each said pair of transistors withalternating conducting periods and short nonconducting dead periodsrespectively between said conducting periods to prevent short circuitingof said consumer load, each said conducting period being accompanied bythe full supply voltage being applied across the nonconducting one ofsaid transistors and the associated one of said auxiliary circuits, saidsupplementary circuit including a choke coil, said choke coil beinglarge enough to substantially reduce the capacitor charging currentthrough the conducting one of said transistors during each of saidconducting periods to prevent damage to said transistors.